FIG. 7 shows an evaporator conventionally used in refrigeration devices such as refrigerators or refrigerated showcases. With reference to FIG. 7, the conventional evaporator 30 comprises two fin groups 32 arranged one above the other at a spacing and each comprising a plurality of plate fins 31 arranged in parallel from left to right, a plurality of straight tubes 33 arranged at a spacing from the front rearward and extending through all the plate fins 31 of the upper and lower fin groups 32, and a plurality of bends 34 interconnecting the respective pairs of adjacent straight tubs 33 so as to pass a refrigerant through all the straight tubes 33 one after another.
The evaporator 30 is fabricated by the following process. First prepared are a plurality of straight tubes 33, plate fins 31 each having holes, and a tube enlarging device comprising a wire and a tube enlarging ball attached to one end of the wire. All the straight tubes 33 are then inserted through the respective holes of each plate fin 31. The wire of the tube enlarging device is thereafter inserted through each tube from one end thereof and pulled at the other end to force the ball through the tube 33 to enlarge the tube 33 and fixedly fit all the plate fins 31 around the tube. The ends of the straight tubes 33 are then welded to opposite ends of U-shaped bends 34 to thereby interconnect all the straight tubes 33 by the bends 34. In this way, the evaporator 30 is fabricated.
Studies are recently under way for the use of hydrocarbon refrigerants which are less likely to destroy the ozone layer and to influence global warming, in refrigerators, refrigerated showcases and like refrigeration devices as substitutes for chlorofluorocarbon refrigerants. Since the hydrocarbon refrigerants are flammable, there is a need to minimize the leakage of the refrigerant.
With the conventional evaporator 30 described above, however, the straight tubes 33 are welded to the U-shaped bends 34, so that the refrigerant is likely to leak from the welded joints.
Accordingly, it is thought that this problem can be overcome by an evaporator prepared from a finned hairpin tube which comprises a hairpin tube, and a plurality of fin groups arranged on the hairpin tube longitudinally thereof at a spacing and each comprising parallel plate fins extending across and fixed to the two straight tube portions of the hairpin tube, by bending the finned hairpin tube zigzag in its entirety at portions thereof having no fin groups.
This evaporator is fabricated by the process to be described below. First prepared are a hairpin tube, a multiplicity of plate fins each having two holes which are spaced apart, and a tube enlarging device comprising a pressure rod and an enlarging mandrel attached to one end of the rod. The two straight tube portions of the hairpin tube are then inserted through the respective holes of each plate fin to thereby arrange the plate fins in parallel into a plurality of unfixed fin groups as spaced apart on the tube portions longitudinally thereof. The mandrel of the tube enlarging device is subsequently forced into the straight tube portions from each open end of the hairpin tube to enlarge the tube portions and to fixedly fit the plate fins of each fin group around the tube portions of the hairpin tube, whereby a finned hairpin tube is produced. The finned hairpin tube is thereafter bent into a zigzag form in its entirety at portions thereof having no fin groups. In this way, the evaporator is fabricated.
The evaporator fabricated by this process has no joints formed in the hairpin tube of the finned hairpin tube, so that no leakage of the refrigerant occurs unlike the evaporator 30 shown in FIG. 7. However, the evaporator is less effective for achieving an improved refrigeration efficiency by an increase in heat transfer area since the plate fins are merely so sized as to extend across the two straight tube portions.
An object of the present invention is to overcome the foregoing problems and to provide a heat exchanger which is capable of exhibiting the desired refrigeration performance with the leakage of refrigerant diminished when used as an evaporator in refrigeration devices.